The present invention relates to irrigation systems for communicating fluid to a surgical site within a body. More specifically it relates to irrigation systems having a valve arrangement capable of safely pressurizing the surgical site under a wide range of flow conditions.
Surgery on interior portions of the body are increasingly often being performed with the aid of endoscopic techniques. In such procedures, a scope cannula is inserted into the body to provide access to the surgical site. An endoscope is inserted into the scope cannula which allows viewing of the surgical site. Surgery is performed with instruments passed through an adjacent cannula or in some instances through the scope cannula or in still other instances through the endoscope itself. In order to facilitate the viewing of the surgical site and the use of the instruments, the tissues surrounding the surgical site are distended by fluid pressure. Typically the scope cannula comprises a dual lumen cannula having an inflow port connected to a first lumen for conducting fluid from a pump to the surgical site and an outflow port connected to a second lumen for conducting fluid away from the surgical site. By these means, a fluid flow loop is established consisting of fluid flowing from the pump through the inflow port, through the first lumen of the scope cannula into the surgical site, and then from the surgical site through the second lumen of the scope cannula and out the outflow port.
In order to distend the surgical site, more fluid is pumped into the surgical site than is allowed to drain from it thereby producing a positive distension pressure at the surgical site. Once a desired pressure is achieved, it is maintained by pumping fluid into the surgical site at the same rate that fluid leaves the surgical site. In practice, this is accomplished by modulating the pump output in order to control a pressure measured at a particular location in the fluid flow circuit.
An exemplary irrigation system is taught in U.S. Pat. No. 5,152,746 wherein a monitor line communicates the fluid pressure within the scope to a pump controller to modulate the pump to maintain the pressure within desirable and safe limits. For added safety, a safety relief valve is included in communication with the inflow line between the pump and the inflow port. When the pressure at the safety relief valve exceeds a safety relief valve activation pressure, the safety relief valve operates to discharge fluid to lower the pressure in the inflow line.
In use, the surgeon may desire to vary the amount of flow through the fluid circuit to accomplish different purposes. In one situation he may stop the outflow from the circuit in order to avoid agitation of the tissues being examined or to preserve the irrigation fluid being used. He may also wish to limit the amount of discharged fluid that must be disposed of. This situation is referred to as a static condition. The pump operates only to replenish the small amount of fluid which inevitably leaks from the circuit but there is little flow through the circuit. In the static condition, the pressure throughout the circuit is equal and therefore the pressure at the safety relief valve is equal to the pressure at the surgical site.
In another situation, the surgeon may want full flow through the circuit in order to actively flush away debris generated by the surgical procedure in order to cleanse the surgical site and facilitate his view through the scope. In this condition, or flow condition, the outflow port is fully open and the pump operates to replenish the fluid that flows through the outflow port and thereby maintain the desired distension pressure. The maximum flow through the system, for a predetermined distension pressure, will occur when the outflow port is fully open.
However, in the flow condition, the pressure measured at two points in the fluid circuit will be different. The difference, or back pressure, represents the incremental pressure required to overcome frictional resistance to the fluid flowing from the first point in the circuit to the second point in the circuit. Back pressure generally increases with increasing circuit length, decreasing tubing diameter, and increasing flow rate among other factors. When there is no flow there is no back pressure and the pressure at the two measurement points will be the same. When there is flow there will be back pressure. Therefore, in the flow condition, the pressure at the safety relief valve will be greater than the pressure at the surgical site. The difference is the back pressure which results from the fluid circuit geometry and the flow rate.
For prior art systems, this back pressure prevents the system from achieving the maximum safe pressure at the surgical site in the flow condition and thus prevents optimal distension. This is because the safety relief valve activation pressure must be set at the maximum safe pressure for the surgical site in order to protect the patient in the static condition. In the flow condition, back pressure causes the safety relief valve to reach its activation pressure before the surgical site reaches the maximum safe pressure. This effect, lowering of the maximum achievable distension pressure, increases as the flow rate through the circuit increases.